10,075 research outputs found
Herschel-PACS Measurements of Nitrogen Enrichment in Nebulae around Wolf-Rayet Stars
For three nebulae that have early-WN Wolf-Rayet exciting stars, NGC 6888, WR
8 and Abell 48, we have obtained Herschel-PACS line scans of the [N III] 57 um
and [O III] 88 micron lines, along with the 122 and 205 micron lines of [N II].
From the former two lines we have derived N/O abundance ratios,
equal to the overall N/O ratio under a wide range of nebular conditions. We
find that all of the nebulae observed possess significant nitrogen enrichment,
with derived N/O ratios greater than solar. The two nebulae with massive
Wolf-Rayet exciting stars, NGC 6888 and WR8 are found to have N/O ratios that
are enhanced by factors of 7 - 10 relative to the solar N/O ratio, consistent
with an origin as material ejected just before the onset of the Wolf-Rayet
phase. The other nebula, Abell 48, has recently been reclassified as a member
of the rare class of three planetary nebulae that have early-WN central stars
and are not of Peimbert Type I. We derive a nebular N/O ratio for it that is a
factor of 4 enhanced relative to solar and slightly above the range of N/O
values that have been measured for the other three members of its [WN]
planetary nebula class.Comment: 11 pages, 5 figures, MNRAS accepte
PAHs and star formation in the HII regions of nearby galaxies M83 and M33
We present mid-infrared (MIR) spectra of HII regions within star-forming
galaxies M83 and M33. Their emission features are compared with Galactic and
extragalactic HII regions, HII-type galaxies, starburst galaxies, and
Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and
M83 HII regions lie in between Seyfert/LINER galaxies and HII-type galaxies in
the 7.7/11.3 - 6.2/11.3 plane, while the different sub-samples exhibiting
different 7.7/6.2 ratios; (ii) Using the NASA Ames PAH IR Spectroscopic
database, we demonstrate that the 6.2/7.7 ratio does not effectively track PAH
size, but the 11.3/3.3 PAH ratio does; (iii) variations on the 17 m PAH
band depends on object type; however, there is no dependence on metallicity for
both extragalactic HII regions and galaxies; (iv) the PAH/VSG intensity ratio
decreases with the hardness of the radiation field and galactocentric radius
(Rg), yet the ionization alone cannot account for the variation seen in all of
our sources; (v) the relative strength of PAH features does not change
significantly with increasing radiation hardness, as measured through the
[NeIII]/[NeII] ratio and the ionization index; (vi) We present PAH SFR
calibrations based on the tight correlation between the 6.2, 7.7, and 11.3
m PAH luminosities with the 24 m luminosity and the combination of
the 24 m and H luminosity; (vii) Based on the total luminosity
from PAH and FIR emission, we argue that extragalactic HII regions are more
suitable templates in modeling and interpreting the large scale properties of
galaxies compared to Galactic HII regions.Comment: 26 pages, 24 figures, 6 tables. Accepted for publication in MNRA
Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNbO in a transverse field: Geometric frustration and quantum renormalization effects
The quasi-one-dimensional (1D) Ising ferromagnet CoNbO has recently
been driven via applied transverse magnetic fields through a continuous quantum
phase transition from spontaneous magnetic order to a quantum paramagnet, and
dramatic changes were observed in the spin dynamics, characteristic of weakly
perturbed 1D Ising quantum criticality. We report here extensive single-crystal
inelastic neutron scattering measurements of the magnetic excitations
throughout the three-dimensional (3D) Brillouin zone in the quantum
paramagnetic phase just above the critical field to characterize the effects of
the finite interchain couplings. In this phase, we observe that excitations
have a sharp, resolution-limited line shape at low energies and over most of
the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the
full bandwidth along the strongly dispersive chain direction and resolve clear
modulations of the dispersions in the plane normal to the chains,
characteristic of frustrated interchain couplings in an antiferromagnetic
isosceles triangular lattice. The dispersions can be well parametrized using a
linear spin-wave model that includes interchain couplings and further neighbor
exchanges. The observed dispersion bandwidth along the chain direction is
smaller than that predicted by a linear spin-wave model using exchange values
determined at zero field, and this effect is attributed to quantum
renormalization of the dispersion beyond the spin-wave approximation in fields
slightly above the critical field, where quantum fluctuations are still
significant.Comment: 11 pages, 6 figures. Updated references. Minor changes to text and
figure
Characterizing upward lightning with and without a terrestrial gamma-ray flash
We compare two observations of gamma-rays before, during, and after lightning
flashes initiated by upward leaders from a tower during low-altitude winter
thunderstorms on the western coast of Honshu, Japan. While the two leaders
appear similar, one produced a terrestrial gamma-ray flash (TGF) so bright that
it paralyzed the gamma-ray detectors while it was occurring, and could be
observed only via the weaker flux of neutrons created in its wake, while the
other produced no detectable TGF gamma-rays at all. The ratio between the
indirectly derived gamma-ray fluence for the TGF and the 95% confidence
gamma-ray upper limit for the gamma-ray quiet flash is a factor of
. With the only two observations of this type providing such
dramatically different results -- a TGF probably as bright as those seen from
space and a powerful upper limit -- we recognize that weak, sub-luminous TGFs
in this situation are probably not common, and we quantify this conclusion.
While the gamma-ray quiet flash appeared to have a faster leader and more
powerful initial continuous current pulse than the flash that produced a TGF,
the TGF-producing flash occurred during a weak gamma-ray "glow", while the
gamma-ray quiet flash did not, implying a higher electric field aloft when the
TGF was produced. We suggest that the field in the high-field region approached
by a leader may be more important for whether a TGF is produced than the
characteristics of the leader itself.Comment: 21 pages, 6 figures, accepted for publication by the Journal of
Geophysical Research - Atmosphere
Economies of Size for Conventional Tillage and No-till Wheat Production
Production costs and economies of size for both conventional tillage and no-till wheat production were determined. The reduction in the price of glyphosate after the patent expired improved the relative economics of no-till for continuous monoculture winter wheat. Production costs differ across farm size and by production system.Crop Production/Industries,
Neutron scattering study of a quasi-2D spin-1/2 dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure
We report inelastic neutron scattering study of a quasi-two-dimensional S=1/2
dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure. The
spin gap {\Delta} becomes softened with the increase of the hydrostatic
pressure up to P= 9.0 kbar. The observed threefold degenerate triplet
excitation at P= 6.0 kbar is consistent with the theoretical prediction and the
bandwidth of the dispersion relation is unaffected within the experimental
uncertainty. At P= 9.0 kbar the spin gap is reduced to 0.55 meV from 1.0 meV at
ambient pressure.Comment: 4 pages, 5 figure
Herschel PACS and SPIRE spectroscopy of the Photodissociation Regions associated with S 106 and IRAS 23133+6050
Photodissociation regions (PDRs) contain a large fraction of all of the
interstellar matter in galaxies. Classical examples include the boundaries
between ionized regions and molecular clouds in regions of massive star
formation, marking the point where all of the photons energetic enough to
ionize hydrogen have been absorbed. In this paper we determine the physical
properties of the PDRs associated with the star forming regions IRAS 23133+6050
and S 106 and present them in the context of other Galactic PDRs associated
with massive star forming regions. We employ Herschel PACS and SPIRE
spectroscopic observations to construct a full 55-650 {\mu}m spectrum of each
object from which we measure the PDR cooling lines, other fine- structure
lines, CO lines and the total far-infrared flux. These measurements are then
compared to standard PDR models. Subsequently detailed numerical PDR models are
compared to these predictions, yielding additional insights into the dominant
thermal processes in the PDRs and their structures. We find that the PDRs of
each object are very similar, and can be characterized by a two-phase PDR model
with a very dense, highly UV irradiated phase (n 10^6 cm^(-3), G
10^5) interspersed within a lower density, weaker radiation field phase
(n 10^4 cm^(-3), G 10^4). We employed two different numerical
models to investigate the data, firstly we used RADEX models to fit the peak of
the CO ladder, which in conjunction with the properties derived yielded
a temperature of around 300 K. Subsequent numerical modeling with a full PDR
model revealed that the dense phase has a filling factor of around 0.6 in both
objects. The shape of the CO ladder was consistent with these components
with heating dominated by grain photoelectric heating. An extra excitation
component for the highest J lines (J > 20) is required for S 106.Comment: 20 pages, 10 figures, A&A Accepte
A scalable, high-speed measurement-based quantum computer using trapped ions
We describe a scalable, high-speed, and robust architecture for
measurement-based quantum-computing with trapped ions. Measurement-based
architectures offer a way to speed-up operation of a quantum computer
significantly by parallelizing the slow entangling operations and transferring
the speed requirement to fast measurement of qubits. We show that a 3D cluster
state suitable for fault-tolerant measurement-based quantum computing can be
implemented on a 2D array of ion traps. We propose the projective measurement
of ions via multi-photon photoionization for nanosecond operation and discuss
the viability of such a scheme for Ca ions.Comment: 4 pages, 3 figure
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